Use of IP address blocks with default interfaces in a router

ABSTRACT

The use of IP address blocks with default interfaces in a router is described herein. According to one embodiment, an exemplary method includes in response to a request for a first IP address received from a first client at an interface of the network element, assigning the first client an IP address from a first block of IP addresses dynamically allocated from an IP address provider separated from a pool of statically preassigned IP addresses, if there is no IP address remained unassigned in the pool of statically preassigned IP addresses, and advertising reachability information in a network with respect to the first block of the IP addresses dynamically allocated from the IP address provider, such that other entities of the network are aware of the first block of the IP addresses. Other methods and apparatuses are also described.

This application claims the benefit of U.S. Provisional Application No.60/516,200, filed Oct. 31, 2003, which is hereby incorporated byreference.

FIELD OF THE INVENTION

The present invention relates generally to communications. Moreparticularly, this invention relates to a use of IP address blocks withdefault interfaces in a router.

BACKGROUND OF THE INVENTION

In the field of communications, the need for high-speed transmission ofdata, including video and audio, has continued to increase. Moreover,there has been an increase in the selection of services by which userscan connect to a network, such as the Internet. Specifically, InternetService Providers (ISPs) may allow for connectivity to the Internetthrough lower-speed connections at different rates, such as 56kilobits/second, by employing a Plain Old Telephone Service (POTS) line.Other choices for connection, which are at higher speeds, into a networkcan include Integrated Services Digital Network (ISDN), DigitalSubscriber Line (DSL) service, and cable modem service over a RadioFrequency (RF) cable line. Further, other types of content providers mayenable a subscriber to receive different types of media, such as a videostream, audio stream, etc.

In a typical DSL network, a network element supports a wide variety offeatures to facilitate the management, allocation and distribution of IPaddresses. Normally, the subscriber profile can be configured locally onthe network element or can be retrieved from a remote server (e.g., aRADIUS (remote access dial in user server)). A subscriber profiledetermines how an IP address (and optionally the associated route forthe subscriber LAN) would be provided to a certain subscriber.Similarly, a cable modem user uses a DHCP server to allocate IPaddresses for the clients.

Traditionally, the IP addresses provided to the subscribers must existwithin the IP address/subnet “boundary” of an interface. That is, aninterface must “contain” or “subtend” the IP addresses that are beingprovided to subscribers. The network element only has to advertise(e.g., perform network broadcast) for the interfaces (not each IPaddress assigned to ech interface). However, since those IP addressesare preprovisioned in relatively large blocks, it is not uncommon forthem to be unused IP addresses (an IP address assigned a subnet that isnot assigned to a subscriber due to a lack of demand cannot be used byanother router). This is unfortunate because IP addresses can be ascarce resource.

When the IP addresses are not adequate, a default interface, alsoreferred to as interface of last resort, may be used to provideadditional IP addresses. In the default interface configuration, the IPaddresses provided to the subscribers do not have to exist within IPaddress/subnet boundary of an interface. That is, the IP addressesprovided to the subscribers are decoupled from the interface(s)definition. This allows a single IP subnet to be fully allocated and tobe shared across multiple routers. However, the network element has toperform network broadcast for each IP address handed out as an interfaceof last resort. As a result, large amount of network traffic isgenerated. In addition, the IP address provider (e.g., DHCP or RADIUS)has to handle each individual IP address request as a last resort, whichleads to a heavy load on the IP address provider.

Typically, there are two major approaches to provide IP addresses to thesubscribers, traditional approach and a default interface (interface oflast resort) approach.

Traditional Approach

This approach is more appropriate for a centralized aggregation model(e.g., BRAS). Utilizing the traditional approach, the following methodsare supported on a network element to manage/allocate IP addresses tothe subscribers:

-   -   1) Static Bridge-1483 encapsulated subscribers: Static IP        addresses are allocated to these subscribers.        Subscriber-specific static routes (usually for the subscriber        LAN) can also be assigned.    -   2) DHCP Bridge-1483 subscribers: IP addresses are allocated to        these subscribers utilizing an external or on-board DHCP server.    -   3) PPP-encapsulated subscribers: A statically or dynamically        allocated IP address can be assigned per PPP session. A        subscriber-specific static route (usually for the subscriber        LAN) can also be assigned at session bring-up time.    -   4) Route-1483 encapsulated subscribers: Static IP addresses are        allocated to these subscribers. A subscriber-specific static        route (usually for the subscriber LAN) can also be assigned.

In all cases above, an IP interface is configured on the network elementthat “contains” the IP addresses that are being assigned to thesubscribers. For example, following interface may be defined on thenetwork element, or if multiple virtual routers are supported, in agiven context (a virtual router or a physical router):

-   -   Interface Subscribers    -   IP address 10.10.10.1 255.255.255.0        The IP addresses assigned to the subscribers (whether static,        DHCP or PPP) fall within a range of 10.10.10.2-10.10.10.254. In        summary, the IP addresses provided to the subscribers are        dependent on the interface(s) configured on the network element.        Default Interfaces or Interfaces of Last Resort

This approach is more commonly used in a more distributed aggregationmodel. It has also been traditionally used for PPP encapsulatedsubscribers in a Remote Access dial-up environment. The defaultinterface feature of the network element would provide the capability todecouple IP address assigned to a session from the interface IPaddress/subnet mask definition. The application of this feature toPPP-encapsulated subscribers would be equivalent to the PPP-defaultinterface feature on the network element. Default Interfaces areapplicable to both PPP-encapsulated and CLIPs-encapsulated subscribers.

The default interface for PPP subscribers may be configured to providePPP sessions an interface to which they can bind in case no other validinterface exists (e.g., a valid interface is one whose IP subnet“contains” the IP address of the subscriber) on a system. Hence also thename: “interface of last resort”. Normally, a PPP session that cannotbind to an interface (due to lack of an interface with a valid matchingIP range) simply fails the binding. With the use of default interfaces,this PPP session will instead bind to the interface designated as a“default” interface. The default interface in this instance acts as aninterface of last resort. By using such a design, there is norequirement to have all subscribers terminated on a single routerinterface be assigned addresses from a common IP subnet. This allows foran IP subnet to be shared across many router devices. This allows theservice provider to more fully utilize the IP address space allocated tothem, as there are no wasted addresses due to allocation inefficiencies(however, IP addresses assigned to a subnet interface may still gounused depending on demand). It also allows the service provider tobuild redundancy into the access network, provided there is a means toreroute subscriber sessions to a standby router. The default interfaceworks in a similar manner as described above except that the serverinvolved here would be the DHCP server.

SUMMARY OF THE INVENTION

The use of IP address blocks with default interfaces in a router isdescribed herein. According to one embodiment, an exemplary methodincludes in response to a request for a first IP address received from afirst client at an interface of the network element, assigning the firstclient an IP address from a first block of IP addresses dynamicallyallocated from an IP address provider separated from a pool ofstatically preassigned IP addresses, if there is no IP address remainedunassigned in the pool of statically preassigned IP addresses, andadvertising reachability information in a network with respect to thefirst block of the IP address dynamically allocated from the IP addressprovider, such that other entities of the network are aware of the firstblock of the IP addresses. Other methods and apparatuses are alsodescribed.

DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings in which likereferences indicate similar elements.

FIG. 1 is a diagram illustrating a typical network infrastructure.

FIG. 2 is a diagram illustrating an exemplary network infrastructureaccording to one embodiment of the invention.

FIG. 3 is a flow diagram illustrating an exemplary process forprocessing a request for an IP address according to one embodiment ofthe invention.

FIG. 4 is a flow diagram illustrating an exemplary process forprocessing a request for releasing an IP address according to oneembodiment of the invention.

FIG. 5 is a block diagram illustrating an exemplary data structure whichmay be used in one embodiment of the invention.

FIG. 6 is a block diagram illustrating an exemplary IP addressconfiguration according to one embodiment of the invention.

DETAILED DESCRIPTION

The use of IP address blocks with default interfaces in a router isdescribed herein. In the following description, numerous details are setforth to provide a more thorough explanation of the present invention.It will be apparent, however, to one skilled in the art, that thepresent invention may be practiced without these specific details. Inother instances, well-known structures and devices are shown in blockdiagram form, rather than in detail, in order to avoid obscuring thepresent invention.

Some portions of the detailed descriptions which follow are presented interms of algorithms and symbolic representations of operations on databits within a computer memory. These algorithmic descriptions andrepresentations are the means used by those skilled in the dataprocessing arts to most effectively convey the substance of their workto others skilled in the art. An algorithm is here, and generally,conceived to be a self-consistent finite sequence of steps leading to adesired result. The steps are those requiring physical manipulations ofphysical quantities. Usually, though not necessarily, these quantitiestake the form of electrical or magnetic signals capable of being stored,transferred, combined, compared, and otherwise manipulated. It hasproven convenient at times, principally for reasons of common usage, torefer to these signals as bits, values, elements, symbols, characters,terms, numbers, or the like.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated otherwise as apparent from the following discussion,it is appreciated that throughout the description, discussions utilizingterms such as “processing” or “computing” or “calculating” or“determining” or “displaying” or the like, refer to the action andprocesses of a computer system, or similar electronic computing device,that manipulates and transforms data represented as physical(electronic) quantities within the computer system's registers andmemories into other data similarly represented as physical quantitieswithin the computer system memories or registers or other suchinformation storage, transmission or display devices.

The invention also relates to one or more different apparatuses forperforming the operations herein. This apparatus may be speciallyconstructed for the required purposes (e.g., software, hardware, and/orfirmware, etc.), or it may comprise a general purpose computerselectively activated or reconfigured by a computer program stored inthe computer. The instructions of such software, firmware, and computerprograms may be stored in a machine readable medium, such as, but is notlimited to, any type of disk including floppy disks, optical disks,CD-ROMs, and magnetic-optical disks, read-only memories (ROMs), randomaccess memories (RAMs), erasable programmable ROMs (EPROMs),electrically erasable programmable ROMs (EEPROMs), magnetic or opticalcards, electrical, optical, acoustical or other forms of prorogatedsignals (e.g., carrier waves, infrared signals, etc.) or any type ofmedia suitable for storing electronic instructions.

The algorithms and displays presented herein are not inherently relatedto any particular computer or other apparatus. Various general purposesystems may be used with programs in accordance with the teachingsherein, or it may prove convenient to construct more specializedapparatus to perform the required method steps. The required structurefor a variety of these systems will appear from the description below.In addition, the present invention is not described with reference toany particular programming language. It will be appreciated that avariety of programming languages may be used to implement the teachingsof the invention as described herein.

An Exemplary Network

FIG. 1 is a block diagram of a network configuration according to oneembodiment of the invention. In one embodiment, no IP address block(s)is needed to be pre-assigned. The addresses can be assigned tosubscribers with no regard for the network element on which thatsubscriber's session is terminated. In the example shown in FIG. 1, thesubscriber session can be terminated on any one of the three units101-103, in spite of the fact that there is no valid matching interfaceon any of the systems 101-103.

Referring to FIG. 1, according to one embodiment, as an example, fournetwork element units 101-104 are set up on an Ethernet segments.However, it is not limited so. Units 101-104 are configured as accesssystems with default interfaces. Unit 104 serves as the upstream router.The backbone interconnecting the four units may be a broadcastmulti-access network, such as, for example, an Ethernet.

Note that the subscriber's address has no corresponding interfaces thatwould normally be considered a “match”. With this configuration, asubscriber can be terminated on any of the three network element units:A, B or C with an address in any of the 10.0.0.1, 10.0.1.1 and 10.0.2.1subnets. All subscribers bound to the default interface appear to unit Dto be directly connected to the backbone network.

Once binding is successful, the backbone router needs to be able to sendpackets to the newly bound subscriber. One way is to distribute hostroutes via an Interior Gateway Protocol, such as OSPF or IS-IS, or anexterior gateway protocol such as BGP. A second way is to use proxy-ARPin conjunction with secondary addressing on the aggregation router.

The first method involving the use of routing protocols to distributehost routes. In this model, an interface is flagged as ‘default’ and allsubscribers with no matching valid interfaces bind to this defaultinterface. The network element can then be configured to distribute allthe reachability information in the form of host routes in a routingprotocol. The backbone learns the existence and location of each of thehosts by means of the routing protocol. Routes to hosts that bind to thedefault interface are treated as subscriber routes. Injecting theseroutes into the routing protocol requires the administrator to configureredistribution of subscriber routes. One of the advantages of thisapproach is that there is no requirement to run Proxy-ARP and theadministrator has full Layer 3 visibility into each flow at all times.The individual PPP session's IP address will appear as a host entry inthe routing table where the session terminates and can be redistributedinto a dynamic routing protocol at that point. Proper network designwould have a common upstream router summarize these host routes prior toadvertising the routes into the backbone.

For the second method, in addition to the default designation, proxy ARPmay be configured on another interface to provide IP reachability.Referring to FIG. 1, this is done on the backbone interface for eachnetwork element (LAN-A, LAN-B and LAN-C). Interfaces with this commandenabled will Proxy-ARP (i.e. generate ARP responses for ARP requests)for any IP address assigned to a PPP session terminated on the defaultinterface on that unit. The backbone router is configured with secondaryaddresses on the B-RAS/router facing side of the device. An exemplaryconfiguration for unit D of FIG. 1 may be defined as follows:

Configuration of Unit D:

-   -   interface RAS        -   ip address 192.168.0.254 255.255.255.0        -   ip address 10.0.0.1 255.255.255.0 secondary        -   ip address 10.0.1.1 255.255.255.0 secondary        -   ip address 10.0.2.1 255.255.255.0 secondary        -   ip arp arpa            This setup allows users to complete their PPP sessions on            any of multiple network element units without concern for            the specific unit on which their session terminates.            Exemplary Address Microblocking

Method 1 above involving the use of routing protocols to distribute hostroutes is the more commonly used approach to deal with defaultinterfaces. This creates a problem in that a large number of host routes(/32s) have to be carried in the routing protocol. This number could beas many as 500,000 routes in a single region. While sometimes this levelof scaling is not a problem on the network element, it could causeproblems on other routers in the network. Reducing the number of routescarried by the routing protocol is highly desirable. A second issue withrouting is that subscribers logging on and off the network results infrequent routing protocol updates which effects the backbone routingconvergence and quality. Devices on the network other than the networkelement may also have issues related to the handling of frequent routeupdates and with having to frequently rerun their algorithms to achieveconvergence.

According to one embodiment, one approach to address this problem is viathe use of address microblocks when performing address allocation on anIP address provider or supplier, such as, for example, a RADIUS or DHCPserver, according to one embodiment. FIG. 2 is a block diagramillustrating an exemplary network configuration according to oneembodiment of the invention. In one embodiment, exemplary networkconfiguration 200 includes, but not limited to, one or more networkelements 201 and 211, where each network element receives one or moresubscribers, such as subscribers 202 and 203 via access network 204 arequest for an IP address, which may be handled through one or morerouters 207, which may be contexts or virtual routers or physicalrouters of network element 201. Typically, prior to servicingsubscribers 202 and 203, network element 201 has been staticallyassigned a range of IP address to form one or more preallocatedinterfaces, also referred to as preassigned IP addresses 208, from an IPaddress provider or supplier 206 through backbone, such as, for example,a DHCP server or a RADIUS. When network element 201 receives such arequest, typically network element 201 hands out an IP address to theclient from the preassigned IP address pool 208, or alternatively, fromone or more microblocks of IP addresses 209, which have been dynamicallyassigned to network element 201 as a default interface or an interfaceof last resort, when no more preassigned IP address is available or anAAA process (performed by a RADIUS) could not match the user informationwith one of the preallocated interfaces. When the client release an IPaddress, network element 201 releases the respective IP address back topreassigned IP address pool or microblock IP address pool. Once all ofthe IP addresses in microblock IP address pool have been released,network element 201 may release the whole microblock of IP addressesback to IP address provider 206 for future use. The released microblockof IP addresses may be reused by other network elements, such as networkelement 211, when another network element needs to allocate a defaultinterface for its clients. As a result, multiple network elements (e.g.,network elements 201 and 211) may share the same pool of IP addressesprovided by IP address provider 206 for their respective defaultinterface.

In one embodiment, the RADIUS or DHCP server (whichever is being used toperform address allocation, depending on the encapsulation in use)allocates addresses to the router in microblocks. For example, a RADIUSserver capable of supporting address microblocking will perform at leastone of the following operations upon receiving a receiving anauthentication request from a certain router:

-   -   Return an address for the subscriber session being terminated on        that router    -   Return a VSA (vendor specific attribute) in the Authentication        response that informs the router that: a) address microblocking        is in use on the RADIUS server, and b) the block size used in        microblocking    -   Allocate a block of adjacent addresses out of a microblock (e.g.        size 8) to be returned when responding to future authentication        requests from that router.

This causes each router to be assigned microblocks of IP addresses. Inone embodiment, the subscriber routes assigned in this manner can betagged as a special route type, such as, for example, subscriber-summaryroutes. The special route type can be used in enforcing policy decisionswhen distributing routes. When these types of routes are distributed viathe routing protocol, the summary route with the specified microblockingprefix length is advertised. If a router does not understand themicroblocking RADIUS VSA, it should ignore the attribute and distribute/32 addresses. The router will also black-hole all packets addressed toaddresses in the micro-block that have yet to be assigned to asubscriber. Thus for example, using a microblock size of 8 could reducethe number of routes distributed by a router via the routing protocol byas much as a factor of 8.

In the case of DHCP and CLIPs subscribers, according to one embodiment,the relay-agent or DHCP proxy may send an Option 82 field indicating amicroblocking capability when relaying a request. In a particularembodiment, this would include a sub-option of “microblocking” withprefix-length of zero. The DHCP server upon receipt of this Option 82field (if it supports microblocking) may fill in the sub-option with thecorrect prefix-length. If the relay-agent does not send this sub-optionor option 82, then the DHCP server does not need to send thisinformation in its response.

When there is a requirement to support changes in block size on the DHCPor RADIUS server, according to one embodiment, intelligence may be builtinto fragment address blocks only as the block of addresses is entirelyfreed up, either via DHCP lease expiration or via subscribers loggingoff and terminating their sessions.

FIG. 3 is a flow diagram illustrating an exemplary process forprocessing a request for an IP address according to one embodiment ofthe invention. Referring to FIGS. 2 and 3, at block 301, when networkelement 201 receives a request for an IP address from a client, networkelement 201 determines whether there is a preassigned IP addressavailable from preassigned IP address pool 208. If there is at least onepreassigned IP address from the preassigned IP address pool 208, atblock 302, network element 201 assigns an IP address from thepreassigned IP address pool 208 to the client.

If there is no more preassigned IP address available, network element201 determines whether there is at least one IP address in an existingmicroblock of IP address, which previously dynamically allocated tonetwork element 201 as a default interface. If there is at least one IPaddress from an existing microblock IP address pool 209, at block 304,network element 201 assigns an IP address out of microblock IP addresspool 209. If there is no more IP address available from microblock IPaddress pool 209 or there is no microblock IP address ever allocated, atblock 303, network element 201 may send a request to IP address provider206 via backbone for a microblock of IP addresses and assign one of theIP addresses in the newly allocated block to the client. Thereafter, atblock 305, network element 201 may advertise the reachabilityinformation with respect to the block of IP addresses.

It will be appreciated that address microblocking and default interfacescan also be used together with the subscriber binding capability on thenetwork element to provision business customers via unnumberedinterfaces on the network element. The RADIUS server can be used toinform the network element about the subscribers IP networks via theFramed-IP-Route attribute. These routes are then installed in the routetable as being reachable via the unnumbered default interface.

FIG. 4 is a flow diagram illustrating an exemplary process for releasingan IP address according to one embodiment. Referring to FIGS. 2 and 4,at block 401, when network element 201 receives a request for releasingan IP address previously allocated to a client, network element 201determines whether the respective IP address was allocated frompreassigned IP address pool 208. If the respective IP address wasallocated from preassigned IP address pool 208, at block 402, networkelement 201 releases the IP address back to preassigned IP address pool208 for future use. If the respective IP address was not allocated frompreassigned IP address pool 208, at block 403, network element 201releases the IP address back to the microblock of IP addresses 209 whereit belongs. In addition, network element 201 determines whether all IPaddresses in the corresponding microblock of IP addresses have beenreleased. If so, at block 404, network element 201 releases the wholemicroblock of IP addresses back to IP address provider 206 and at block405, network element 201 stops advertising the reachability informationin the network with respect to the released microblock of IP addresses.Other operations may be included.

In one embodiment, when IP address provider 206 allocates a microblockof IP addresses to network element 201, the microblock of IP addressesmay be defined to include a start address and the length of themicroblock. FIG. 5 is a block diagram illustrating data structuresmaintained by a network element and an IP address provider, according toone embodiment. In one embodiment, data structure 501 maintained by anetwork element may include context ID field 503 and microblock addressID field to indicate which microblock IP addresses is being used bywhich context or virtual router. Data structure 501 may be implementedas microblock of IP addresses 209 of FIG. 2. Similarly, an IP addressprovider, such as IP address provider 206, also maintains a datastructure 502 to maintain information regarding microblocks of IPaddresses allocated. In one embodiment, data structure 502 may includemicroblock IP address ID field 505 and network element ID field toindicate which microblock of IP addresses has been assigned to whichnetwork element.

Referring to FIGS. 2 and 5, according to one embodiment, when networkelement 201 receives a microblock of IP addresses from IP addressprovider 206, the microblock includes, but not limited to, a startaddress and a length of the microblock indicating number of IP addressesin the microblock. In one embodiment, the length of the microblock isindicated by a prefix length of network address, similar to a mask, asexemplary IP address 600 shown in FIG. 6.

For example, referring to FIG. 6, the length of the microblock of IPaddresses is specified by a prefixed length of a network address 601indicating how many bits of the address are used for the networkaddress. The remaining bits 602 indicate the length or size of themicroblock. In this example, the remaining bits have 5 bits which may betranslated to 32 IP addresses (included in a microblock). It will beappreciated that other mechanisms may be used to indicate the startaddress and the size of the microblock.

Thus, the use of default interfaces allows for a simple provisioningmodel where scarce IP addresses can be fully utilized. Further the useof microblocking on RADIUS and DHCP servers in conjunction with supporton the router greatly reduces the impact of using such defaultinterfaces on systems and the routing protocols. Address microblockingalso reduces the amount of state that has to be carried in routingprotocols as well as the frequency of updates. All these factors resultin a more scalable, flexible and stable network.

Thus, the use of IP address blocks with default interfaces in a routerhas been described herein. In the foregoing specification, the inventionhas been described with reference to specific exemplary embodimentsthereof. It will be evident that various modifications may be madethereto without departing from the broader spirit and scope of theinvention as set forth in the following claims. The specification anddrawings are, accordingly, to be regarded in an illustrative senserather than a restrictive sense.

1. A method performed by a network element, comprising: in response to arequest for a first IP address received from a first client at aninterface of the network element, assigning the first client an IPaddress from a first block of IP addresses dynamically allocated from anIP address provider separated from a pool of statically preassigned IPaddresses, if there is no IP address remained unassigned in the pool ofstatically preassigned IP addresses; and advertising reachabilityinformation in a network with respect to the first block of the IPaddresses dynamically allocated from the IP address provider, such thatother entities of the network are aware of the first block of the IPaddresses.
 2. The method of claim 1, wherein the IP address provider isone of a DHCP server and a RADIUS server coupled to the network elementvia a backbone.
 3. The method of claim 1, wherein the interface is adefault interface of the network element.
 4. The method of claim 1,further comprising: determining whether there is at least one IP addressavailable from the pool of statically preassigned IP addresses; anddynamically allocating the first block of IP addresses from the IPaddress provider if there is no IP address available from the pool ofstatically preassigned IP addresses.
 5. The method of claim 4, furthercomprising: prior to allocating the first block of IP address,determining whether a second block of IP addresses previouslydynamically allocated from the IP address provider in response to aprevious request for an IP address; and assigning an IP address from thesecond block of IP addresses previously allocated without allocating thefirst block of IP addresses if the second block of IP addresses existsand contains at least one IP address available.
 6. The method of claim5, wherein the first block of IP addresses is dynamically allocated onlyif the second block of IP addresses does not exist.
 7. The method ofclaim 5, wherein the first block of IP addresses is dynamicallyallocated only if the second block of IP addresses does not contain atleast one IP address available.
 8. The method of claim 1, furthercomprising: receiving from a second client a request for releasing asecond IP address previously allocated from the first block of IPaddresses; and releasing the second IP address back to the first blockof IP addresses without invoking the IP address provider.
 9. The methodof claim 8, further comprising: receiving a request for allocating athird IP address from a third client; and reassigning the second IPaddress from the first block of IP addresses to the third client withoutinvoking the IP address provider.
 10. The method of claim 8, furthercomprising: determining whether all IP addresses of the first block ofIP addresses have been released; and releasing the first block of IPaddresses back to the IP address provider if all IP addresses of thefirst block of IP addresses have been released.
 11. A machine-readablemedium having executable code to cause a machine to perform a method,the method comprising: in response to a request for a first IP addressreceived from a first client at an interface of the network element,assigning the first client an IP address from a first block of IPaddresses dynamically allocated from an IP address provider separatedfrom a pool of statically preassigned IP addresses, if there is no IPaddress remained unassigned in the pool of statically preassigned IPaddresses; and advertising reachability information in a network withrespect to the first block of the IP addresses dynamically allocatedfrom the IP address provider, such that other entities of the networkare aware of the first block of the IP addresses.
 12. Themachine-readable medium of claim 11, wherein the IP address provider isone of a DHCP server and a RADIUS server coupled to the network elementvia a backbone.
 13. The machine-readable medium of claim 11, wherein theinterface is a default interface of the network element.
 14. Themachine-readable medium of claim 11, wherein the method furthercomprises: determining whether there is at least one IP addressavailable from the pool of statically preassigned IP addresses; anddynamically allocating the first block of IP addresses from the IPaddress provider if there is no IP address available from the pool ofstatically preassigned IP addresses.
 15. The machine-readable medium ofclaim 14, wherein the method further comprises: prior to allocating thefirst block of IP address, determining whether a second block of IPaddresses previously dynamically allocated from the IP address providerin response to a previous request for an IP address; and assigning an IPaddress from the second block of IP addresses previously allocatedwithout allocating the first block of IP addresses if the second blockof IP addresses exists and contains at least one IP address available.16. The machine-readable medium of claim 15, wherein the first block ofIP addresses is dynamically allocated only if the second block of IPaddresses does not exist.
 17. The machine-readable medium of claim 15,wherein the first block of IP addresses is dynamically allocated only ifthe second block of IP addresses does not contain at least one IPaddress available.
 18. The machine-readable medium of claim 11, whereinthe method further comprises: receiving from a second client a requestfor releasing a second IP address previously allocated from the firstblock of IP addresses; and releasing the second IP address back to thefirst block of IP addresses without invoking the IP address provider.19. The machine-readable medium of claim 18, wherein the method furthercomprises: receiving a request for allocating a third IP address from athird client; and reassigning the second IP address from the first blockof IP addresses to the third client without invoking the IP addressprovider.
 20. The machine-readable medium of claim 18, wherein themethod further comprises: determining whether all IP addresses of thefirst block of IP addresses have been released; and releasing the firstblock of IP addresses back to the IP address provider if all IPaddresses of the first block of IP addresses have been released.
 21. Anetwork element, comprising: a processor; and a memory coupled to theprocessor to store instructions, when executed by the processor, causethe processor to in response to a request for a first IP addressreceived from a first client at an interface of the network element,assign the first client an IP address from a first block of IP addressesdynamically allocated from an IP address provider separated from a poolof statically preassigned IP addresses, if there is no IP addressremained unassigned in the pool of statically preassigned IP addresses,and advertise reachability information in a network with respect to thefirst block of the IP addresses dynamically allocated from the IPaddress provider, such that other entities of the network are aware ofthe first block of the IP addresses.
 22. The network element of claim21, wherein the interface is a default interface.
 23. The networkelement of claim 21, wherein the IP address provider is one of RADIUSserver and DHCP server coupled to the network element through abackbone.